In a spectrophotometer, a beam of light of a selected wavelength or frequency is passed through a sample where some of the light is absorbed by the molecules comprising the sample. The light which passes through the sample is received by a light sensitive detector system such as a photometer. The less light energy that is absorbed by this sample results in more light being received by the light detector system. The detector system generates an electrical signal of the strength proportional to the intensity of the light it receives. The output of the light detector system, for example one utilizing a photomultiplier tube, is generally an analog current signal proportional to the light intensity received, which thus is proportional to the light transmittance of the sample. It is the light transmittance of the sample which is of interest in that it indicates light absorbance by the sample which can be used to determine sample composition.
The light detector system generally has an amplifier such as an operational amplifier to convert the analog current signal from the light detector to an analog DC voltage signal. The DC voltage signal is processed by additional electronics and applied to a display, such as a chart recorder, to provide a visual and/or permanent record of the sample light transmittance, i.e. absorbance (absorbance=1/transmittance) at a selected light wavelength or through a wavelength scan.
The additional electronics may comprise, for example, digital to analog converters for adjusting the offset and gain of the detector system amplifier(s) to obtain calibration of the instrument analysis function. Log arithmic conversion electronics are provided to convert light transmittance values measured by the light detector system to light absorbance values for direct interpretation of an analysis by a user. A resistor network may be provided through which the signal produced by the light detector system may be selectively altered to provide means for calibration of the electronics for reduced transmittance signals. This is accomplished by directing the detector output through a select resistor of the network and then to the log conversion electronics and to an analog to digital converter (ADC) to generate a digital calibration signal corresponding to incremental absorbance values. Once the signal level for incremental absorbance values has been calibrated, it can be used during sample analysis as a basis for interpolation by the computer to determine the actual light absorbance by a sample. Preferably, calibration of the electronics at light transmittance levels of 100, 10, 1, and 0.1 percent transmission of light are performed which correspond to selected absorbance values of 0A, 1A, 2A and 3A, respectively, due to the logarithmic relationship between transmission and absorbance. A schematic representation of forms of such calibration and signal electronics are shown in FIGS. 1 and 2. Such electronics and their usage are described in the following U.S. Pat. Nos., 4,436,994 of Van Vliet, et al., 4,310,243 of Brown, et al., 4,300,203 of Brown.
In order to obtain accurate performance of the spectrophotometer in analysis of a sample, it is necessary to perform calibration of the spectrophotometer at least each time a new wavelength of light is selected for analysis. This is a time consuming operation, and in particular can be a burden to a user for sample analysis utilizing more than a single light wavelength which is common to spectrophotometer users.
A need thus exists in the field of scientific instrumentation which utilizes varying wavelength monochromatic light for analysis purposes, to improve the ability and speed with which a user can calibrate his instrument to provide improved accuracy and reproducibility in its function by decreasing the time necessary for calibration so that the instrument is more readily available for use.